1. Field of the Invention
The present invention relates to cold cathode lamps.
2. Description of the Related Art
The schematic cross-sectional view of a conventional cold cathode lamp is shown in FIG. 14. The conventional cold cathode lamp shown in FIG. 14 has internal electrodes 2 and 3 inside a glass tube 1. Parts of the internal electrodes 2 and 3 penetrate the glass tube 1 to protrude outward from the glass tube 1, and they serve as electrode terminals. The glass tube 1 configured as described above is hermetically sealed. The inner wall of the glass tube 1 is coated with fluorescent material. Generally, neon and argon are sealed in the glass tube 1 in the proportion of 95 to 5 or 80 to 20 or in other proportions such that the overall pressure inside the glass tube 1 falls within the range of 5.3×103 to 10.7×103 Pa (≈40 to 80 torr), and a few milligrams of mercury is further sealed in the glass tube 1. Instead of mercury, xenon may be sealed in the glass tube 1.
When a lamp voltage (a voltage between the internal electrodes) reaches a discharge start voltage Vs, a discharge is started. The discharge causes mercury or xenon to generate ultraviolet rays, and the generated ultraviolet rays cause the fluorescent material coated on the inner wall in the glass tube 1 to emit light.
The conventional cold cathode lamp shown in FIG. 14 acts, in terms of its equivalent circuit, as a resistor whose resistance nonlinearly decreases with increasing current. The conventional cold cathode lamp has a nonlinear negative impedance characteristic such as a V-I characteristic shown in FIG. 15 (for example, see patent document 3).
One application of the conventional cold cathode lamp shown in FIG. 14 is a backlight for use in a liquid crystal display device. When the display screen of the liquid crystal display device is large, a plurality of cold cathode lamps are arranged for use in the liquid crystal display device. In this case, if parallel driving is achieved in a plurality of cold cathode lamps, an equal voltage is applied to all the cold cathode lamps, with the result that only one power supply is needed.
Here, consider a case where a plurality of (for example, three) cold cathode lamps in parallel are driven together. The V-I characteristics vary from one cold cathode lamp to another. Thus, the V-I characteristic curves T1 to T3 of the first to third cold cathode lamps are assumed to be as shown in FIG. 16. An equal alternating-current voltage is applied to the first to third cold cathode lamps, and the alternating-current voltage is then stepped up. When the stepped up alternating-current voltage reaches the discharge start voltage VS1 of the first cold cathode lamp, the first cold cathode lamp lights up, and the voltage across the first cold cathode lamp is dropped according to its nonlinear negative impedance characteristic. Since the voltages across the second and third cold cathode lamps are equal to that across the first cold cathode lamp, the above-mentioned alternating-current voltage does not reach the discharge start voltage VS2 of the second cold cathode lamp and the discharge start voltage VS3 of the third cold cathode lamp. That is, when a plurality of cold cathode lamps in parallel are simply driven together, only one cold cathode lamp can be lit. Thus, one power supply is generally provided for each cold cathode lamp so that a plurality of cold cathode lamps are lit. Disadvantageously, however, with such a configuration, as many power supplies as there are cold cathode lamps are required, and this results in a higher cost. It is also disadvantageous in terms of compactness, lightness and cost. Since cold cathode lamps are generally connected to power supplies through wiring harness (also called lead wires) and connectors, the following disadvantages arise. The mounting of cold cathode lamps is time-consuming, and this results in low efficiency with which an illumination device or the like incorporating cold cathode lamps is assembled. The removal of cold cathode lamps is also time-consuming, and this results in low efficiency with which a cold cathode lamp is replaced or an illumination device or the like incorporating cold cathode lamps is disassembled after being disposed of.
To overcome these disadvantages, external electrode fluorescent lamps (EEFLs) are being developed (for example, see patent documents 1 and 2). The schematic cross-sectional view of the external electrode fluorescent lamp is shown in FIG. 17. In FIG. 17, such parts as are found also in FIG. 14 are identified with common reference numerals, and their detailed description will not be repeated. The external electrode fluorescent lamp shown in FIG. 17 differs from the conventional cold cathode lamp shown in FIG. 14 in that the internal electrodes 2 and 3 are removed, and external electrodes 4 and 5 are formed on the ends of the glass tube 1. The glass tube 1 configured as described above is hermetically sealed.
In the external electrode fluorescent lamp shown in FIG. 17, when a lamp voltage (a voltage between the external electrodes) reaches a discharge start voltage Vs′, a discharge is started. The discharge causes mercury or xenon to generate ultraviolet rays, and the generated ultraviolet rays cause the fluorescent material coated on the inner wall in the glass tube 1 to emit light.
Since the interior of the glass tube 1 has a nonlinear negative impedance characteristic, and the external electrodes are insulated from the interior of the glass tube 1 by glass, the external electrode fluorescent lamp shown in FIG. 17 acts, in terms of its equivalent circuit, as a series connected member in which a capacitor is connected to each end of a resistor whose resistance nonlinearly decreases with increasing current. Thus, the external electrode fluorescent lamp shown in FIG. 17, as a whole, has a nonlinear positive impedance characteristic like a V-I characteristic shown in FIG. 18.
Here, consider a case where a plurality of (for example, three) external electrode fluorescent lamps in parallel are driven together. The V-I characteristics vary from one external electrode fluorescent lamp to another. Thus, the V-I characteristic curves T1′ to T3′ of the first to third external electrode fluorescent lamps are assumed to be as shown in FIG. 19. An equal alternating-current voltage is applied to the first to third external electrode fluorescent lamps, and the alternating-current voltage is then stepped up. When the stepped up alternating-current voltage reaches the discharge start voltage VS1′ of the first external electrode fluorescent lamp, the first external electrode fluorescent lamp lights up. Thereafter, as the output of a power supply increases, the alternating-current voltage is increased. Then, when the alternating-current voltage reaches the discharge start voltage VS2′ of the second external electrode fluorescent lamp, the second external electrode fluorescent lamp lights up. Then, when the alternating-current voltage reaches the discharge start voltage VS3′ of the third external electrode fluorescent lamp, the third external electrode fluorescent lamp lights up. That is, even when a plurality of external electrode fluorescent lamps in parallel are simply driven together, all the external electrode fluorescent lamps can be lit.
In an illumination device or the like incorporating the external electrode fluorescent lamps, since the external electrodes are provided on the perimeter of the glass tube, by the elastic action of a holder formed by an elastic metal member (made of, for example, spring steel), the external electrodes of the external electrode fluorescent lamp are held by the holder. Thus, it is possible to supply power to the external electrode fluorescent lamps through the holders. This configuration is advantageous in that the mounting and removal of the external electrode fluorescent lamp is facilitated.    Patent document 1: JP-A-2004-031338    Patent document 2: JP-A-2004-039264    Patent document 3: JP-A-H07-220888 (FIG. 4)    Patent document 4: JP-A-2004-039336    Patent document 5: JP-A-H05-121049    Patent document 6: JP-A-S64-082452    Patent document 7: JP-A-2003-100482    Patent document 8: JP-A-H11-040109    Patent document 9: JP-UM-A-H02-041362    Patent document 10: JP-A-H06-084499
Disadvantageously, however, since the glass interposed between the external electrodes and the space inside the glass tube corresponds to a dielectric sandwiched between the electrodes of a capacitor that is one of the elements in the equivalent circuit of the external electrode fluorescent lamp, charged particles collide with the portions of the inner wall of the glass tube opposite the external electrodes, and the inner wall is locally sputtered. Once the inner wall is sputtered, the capacitance of the sputtered portions is increased, and thus charged particles collide with the sputtered portions in a concentrated manner, with the result that pinholes are formed. This makes it difficult to maintain the sealed condition of the glass tube. As described above, the external electrode fluorescent lamps have poor reliability.